Encyclopedia of Microtonal Music Theory

The "dialogus" of pseudo-Odo

the first use of Roman letter nominals
modulo A B C D E F G as music notation

[Joe Monzo]

Below is a photograph of the section from Chapter 2 which describes (starting at line 6 on the left) the division of the monochord in pythagorean tuning, using the Roman letter nominals A B C D E F G as a modulo-7 system recognizing octave-equivalence for the first time; from the manuscript codex Paris, Bibliothèque Nationale, Fonds Latin 7211, f.107r and 107v.

When you divide the sounds in the middle, you must mark them differently. For example, when you bisect the distance from Γ, instead of Γ, write G; for A bisected, set down a second a; for B, a second ♮ ; for C, a second c; for D, a second d, for E, a second e; for F, a second f; for G, a second g; and for a, a second aa; so that from the middle of the monochord forward, the letters will be the same as in the first part.

In addition, from the sixth step, F, divide into four, and before ♮ , place a second round b; these two are accepted as a single step, one being called the second ninth step, and both are not regularly found in the same melody.

The first is the quaternary division, as from the first step, A, to the fourth, D, so called because it is a division by four; this has four pitches and three intervals, namely, two tones and one semitone.

Therefore, wherever you find two tones and a semitone between two pitches on the monochord, you will discover on trial that the interval formed by these two pitches extends to the very end in quaternary division; for this reason it is called diatessaron, that is, "of four".